The design and production of loudspeakers for stereo audio systems is an art which is constantly changing in an effort to provide a loudspeaker which can simulate a live musical performance as closely as possible. In order to do so, the stereo loudspeakers should provide a sharp stereo image, i.e., a spatial recreation of the distribution of the components of the performance, and a multidirectional coverage of the stereo image such that the image can be perceived by a listener from virtually any position relative to the loudspeakers.
Several factors are vital to the production of a sharp stereo image. These are the listener's ability to determine the direction of a sound source, the utilization of an effective point source, and the elimination of secondary, or phantom, sources (diffraction and unwanted reflections). These tend to blur the point source, creating false imagery, as if another speaker were present.
The crest of the first sound wave reaching the listener's ear is critical in determining the direction of the sound source (and hence, stereo imagery). This is especially true in frequencies upward of around 200 hz., becoming more pronounced as the frequency increases. In frequencies downward from 200 hz., the human ear is incapable of determining the direction of the sound source.
The ideal stereo system is based upon two point sources in order to achieve an ideally sharp stereo image. In physical reality, a true point source is extremely hard to achieve, since sound usually emanates from a large area rather than from a point.
The closest practical means of approximating a point source is derived through use of a boundary controlled virtual image, whereby the construction of the cabinet (later described) allows the listener only to hear certain portions of the virtual image from each listening angle. Thus a pair of such cabinets will maintain a sharp stereo image in a multidirectional coverage throughout the listening area. This allows the listener the freedom of perceiving a sharp stereo image (since he hears only the virtual image) regardless of his position relative to the listening area.
Unwanted secondary sources (which tend to spread the ideal point source) are eliminated by lining the cabinet with acoustically absorbent material. This, in effect, "soaks up" unwanted sound and enhances the stereo image through the elimination of any fuzzy or blurred images. One interesting design approach for attempting to simulate a live performance from a stereo loudspeaker is that embodied in the loudspeaker sold commercially under the tradename of the Bose 901. This loudspeaker contains a plurality of acoustic speakers which are positioned in the loudspeaker enclosure such that one speaker projects sound in a forward direction, i.e., directly toward a listener, and the remainder of the speakers, grouped in two arrays with a slight angular displacement from each other, project sound rearwardly such that projected sound will strike one or more walls and be reflected before reaching a listener. The concept behind this loudspeaker design is that in a live performance in a concert hall or the like, only a relatively small proportion of the sound generated reaches the listener directly as the greater portion is first reflected from the walls, ceiling and floor of the building where the performance is occurring before reaching the listener. Thus, by placing a majority of the acoustic speakers such that projected sound is directed toward a wall and then reflected and by having sound from only one acoustic speaker reaching a listener directly, a live performance within a building is thereby supposedly simulated.
By having the rearwardly directed speakers reflect sound off adjacent walls these loudspeakers tend to create what can be termed a "virtual image", i.e., an audio image of the sound from the rearwardly directed speakers which, to a listener, appears to be emanating from a source located behind the wall at which the speakers are aimed. This virtual image seemingly is located a distance behind the wall that is equal to the distance between the loudspeaker and the wall and on a line normal to the wall through the loudspeaker. The creation of the virtual image thus tends to separate the source of the sound from the loudspeaker itself.
While the above-described loudspeaker operates satisfactorily in many respects, there are certain listening positions where the physical presence of the loudspeaker itself tends to block or impede portions of the reflected sound from the virtual image created by the rearwardly directed speakers thereby producing certain locations where listening is less than optimal. In addition, the provision of the single forwardly directed speaker tends to reduce or blur the virtual image created by the rearwardly directed speakers and tends to degrade the stereo imagery since both direct and reflected sounds are heard by a listener.
It is a purpose of the present invention to allow the listener to hear only the virtual image of a feasibly approximated point source throughout the listening area. This is achieved by maximizing use of the virtual image through elimination of all secondary sources within the speaker cabinet and by allowing the cabinet itself to become pervious to the virtual image. Thus, the past disadvantages associated with reflected sound have not merely been overcome; they have been turned to advantage.